This invention is related to a method for isolating kerogen from rock samples. More particularly, the invention is concerned with a method useful for isolating kerogen in a pressurized reaction cell which provides for the removal of all liquids from the cell without significant loss of sample.
Geochemical analysis has become an important part of petroleum exploration over the last two decades. Of key importance to the understanding of the geochemistry of the depositional setting is the analysis of bitumen and kerogen fractions found in sedimentary rock.
There are numerous definitions of bitumen and kerogen. Bitumen will be defined herein as the fraction of sedimentary organic matter which is soluble in organic solvents at moderate temperatures under about 70.degree. C. Compounds extracted in this manner by chloroform, benzene, methanol-benzene mixture, tetrahydrofuran and others are simple hydrocarbons and more complex products which may be heteroatomic and have a high atomic weight such as resins and asphaltenes. This bitumen should not be confused with road bitumen. Road bitumen is an asphalt preparation. Bitumen as used herein is the organic matter within rocks that is soluble in the usual organic solvents.
Kerogen is a solid form of organic matter found in sedimentary rock that is insoluble in water, non-oxidizing acids and bases, and the usual organic solvents. It thermally degrades in a predictable manner by releasing hydrocarbons and condensing the solid organic structure. Because of its immobility, it can provide direct historical evidence of geological conditions within a stratigraphic sequence.
Characterization of kerogen, the precursor to oil and gas, is essential in determining the potential of a rock unit to generate hydrocarbons. The study of kerogen is accomplished by first isolating the kerogen from its rock matrix, and then studying its morphology by several methods, including transmitted light microscopy, its organic metamorphism or thermal alteration by reflected light microscopy, and its chemistry by elemental, isotopic and pyrolysis methods.
Kerogen must be isolated in such a way that the isolated fraction is as representative as possible of in situ kerogen. Analysis requires the recovery of a sufficient amount of kerogen sample without chemical alteration. A principle objective is to keep the morphology of the kerogen intact and to recover identifiable organic debris.
The isolation of kerogen is a complicated chemical process that involves the use of strong acids and bases that dissolve the rock matrix without modifying the kerogen. The rock sample is first finely ground in order to facilitate reaction with the reagents. The methods now used for kerogen isolation employ the dissolution of silicates by hydrofluoric acid and the dissolution of sulfides, sulphates, carbonates, oxides and hydroxides by hydrochloric acid. The reactions are normally carried out below 70.degree. C., a temperature sufficient to dissolve carbonates, but inadequate to promote oxidation and degradation of the organic matter. For a general discussion of the methods of kerogen isolation and reagents employed, please see Durand, B. and Nicaise, G., "Chapter Two-Procedures for Kerogen Isolation," within Kerogen, edited by Durand, B., Graham & Trotman Ltd, London (1980) p. 35-52.
Generally the acid and base reactions are carried out in open plastic beakers placed in a steam bath to raise reaction temperature. After reaction, the aqueous liquids are removed from the beakers by decanting. The reaction steps are repeated until kerogen isolation is judged to be relatively complete. This usually takes anywhere from two to four weeks depending on the quality requirements of subsequent analyses and the type of rock being dissolved.
The process of decanting as well as the length of the procedure leaves much to be desired. A disadvantage to current isolation techniques is that a percentage of the kerogen is frequently lost during decanting. Second, since aqueous liquids are never completely removed from the beakers, solvated metal and silicate ions are available as reactants to form fluoride precipitates. Once precipitated, these neoformed fluorides are virtually impossible to dissolve and remove without damaging the remaining kerogen. Third, beakers permit exposure to oxygen which creates undesirable oxidation products.
An additional step is often required for the final separation of kerogen from unreacted minerals and precipitates. A high density solution of zinc bromide is frequently mixed with the kerogen to cause the lower density kerogen to float. The floated kerogen is decanted and water washed prior to subsequent analyses. Unfortunately, this causes further fractionation and loss of kerogen which lowers the quality and representativeness of the isolated kerogen sample relative to its in situ form in the original rock.
Other difficulties exist with current kerogen isolation procedures. Dangers to the workers who perform kerogen and bitumen isolation in the standard open beaker method include exposure to hazardous highly concentrated hydrofluoric and hydrochloric acids and bases such as ammonium hydroxide, all of which must be added and decanted manually. Use of such chemicals requires not only protective personal equipment but also engineering controls such as hoods and other vacuum equipment. In addition, chemical solutions containing zinc bromide and organic solvents are environmentally hazardous and require special handling for disposal.